The present invention relates to an optical pickup device and an optical element used for the optical pickup device, and in particular, to an optical pickup device capable of recording and/or reproducing information by the use of a violet laser or lasers other than the violet laser and to an optical element used for the optical pickup device.
Backed by practical application of a short wave red semiconductor laser, there has recently been developed and commercialized DVD (digital versatile disk) representing a high density optical disk which is similar to CD (compact disk) representing a conventional optical disk (also called an optical information recording medium) in terms of a size and has a greater capacity, and it is estimated that an advanced optical disk having higher density will appear in the near future. In the light-converging optical system of an optical information recording and reproducing apparatus (also called an optical pickup device) wherein such advanced optical disk is a medium, a diameter of a spot converged on an information recording surface through an objective lens is required to be small, for the purpose of achieving higher density of recording signals, or for reproducing higher density recording signals. For that purpose, a short wavelength of a laser representing a light source and a higher numerical aperture of the objective lens are required. What is expected in terms of practical use as a short wavelength laser light source is a violet semiconductor laser with a wavelength of about 400 nm.
There have been advanced studies and developments of a high density disk system capable of conducting recording/reproducing by the use of this violet semiconductor laser with a wavelength of about 400 nm. As an example, in the case of the optical disk conducting recording/reproducing information under the specifications of NA 0.85 and wavelength of 405 nm (hereinafter referred to as “high density DVD” in the specification), information of 20-30 GB per one surface can be recorded for the optical disk having a diameter of 12 cm which is the same as DVD (NA 0.6, light source wavelength 650 nm, storage capacity 4.7 GB) in size.
When an objective lens having high NA is made to be a plastic lens in an optical pickup device for high density DVD, there is generated spherical aberration caused by changes in the refractive index resulting from temperature changes, which is a problem. The problem of this kind is caused by a plastic lens which is greater than a glass lens by magnifying power of a two-digit number in terms of a change of the refractive index resulting from temperature changes. Since this temperature aberration is proportional to the fourth power of NA, when an objective lens having NA of 0.85 to be used for high density DVD is made to be a plastic lens, a range of temperatures which can be used for that lens is reduced to be extremely narrow, which is a problem for the practical use of the lens. Further, since a semiconductor laser originally causes a wavelength fluctuation phenomenon which is called a mode hop, it is necessary to control aberration in a converging spot on an information recording surface, even when a mode hop is caused. In addition, there is generally dispersion of oscillation wavelength between individuals of a semiconductor laser, and even in the case of a combination of semiconductor lasers having dispersion to a certain extent and objective lenses, it is necessary to form a converging spot which is appropriate to a level that makes recording/reproducing of information possible. For that purpose, spherical aberration caused by fluctuations of light source wavelength needs to be controlled by some methods or other.
Further, value of the optical pickup device as a product is not sufficient if the optical pickup device cannot conduct more than recording/reproducing information properly for high density DVD. When considering that DVDs having therein various types of information recorded are on the market presently, recording/reproducing information properly for high density DVD only is not enough, and the value of the product as an optical pickup device of an interchangeable type is enhanced, by achieving that recording/reproducing of information can equally be conducted properly for conventional DVDs owned by, for example, a user. From the background mentioned above, it is necessary, with respect to a light-converging optical system used for an optical pickup device of an interchangeable type, to control properly all of aberration deterioration caused by temperature changes, aberration deterioration caused by wavelength fluctuations and aberration deterioration in the case of a mode hop (or chromatic aberration), on the converging spot formed on the information recording surface when recording/reproducing information for high density DVD, and it is necessary to control properly all of aberration deterioration caused by temperature changes, aberration deterioration caused by wavelength fluctuations and aberration deterioration in the case of a mode hop (or chromatic aberration), on the converging spot formed on the information recording surface when recording/reproducing information for conventional DVD. However, it is extremely difficult to satisfy a plurality of aberration conditions with a single light-converging optical system. Nevertheless, if two light-converging optical systems each having an objective lens are provided for converging a violet laser beam and a conventional laser beam separately, an optical pickup device is made to be large in size, and its cost is increased, which is a problem. Incidentally, the structure for converging a violet laser beam and a conventional laser beam with a single light-converging optical system is disclosed in the following patent document, but there is no disclosure about a design to consider temperature characteristics and wavelength characteristics.
(Patent Document 1)